What Causes Severe Menstrual Cramps?

I am having very horrible menstrual cramps and i don't know what to do. This normally wouldn't concern me except I took plan B 3 or 4 weeks ago. I had no side effects, then my period was a week late, and this is the worst I have ever had it. Pain, bleeding, clotting, fever. I have taken so much ibp and it doesn't touch it. should i go to the emergency room?